The role of the amphibian chytridiomycete fungus in population dynamics of the Spotted Tree Frog (Litoria spenceri) a declining riverine species in south-eastern Australia

Graeme Gillespie^{1 and Gerry Marantelli2}

1 Arthur Rylah Institute for Environmental Research, Department of Natural Resources & Environment, PO Box 137, Heidelberg, Vic., Australia 3084. Email: Graeme.Gillespie@nre.vic.gov.au
2 Amphibian Research Centre, P.O. Box 424, Brunswick, Vic., Australia 3056. Email: arc@frogs.org.au

The Spotted Tree Frog Litoria spenceri is listed as endangered nationally and is one of a number if riverine frogs that have declined in recent years. Since 1992 a intensive research and monitoring program has been under way to investigate the nature and causes of population declines. Research has identified a number of threatening processes including predation by brown trout as well as a variety of habitat disturbances. With the exception of one site the species has persisted at low density with a restricted and fragmented distribution. The exception, Bogong Creek, remained at high and consistent density between 1993 and 1996. During the 1995-96 season several dead and one sick frog were located along the stream. By late 1996 only a handful of frogs were detectable along the stream. In 1999 the last individual was removed and placed in captivity.

Examination of freshly dead specimens collected in 1996 revealed the presence of the chytrid fungus. Subsequent examination of toes collected during surveys from 1993 to 1996 only located chytrids in two toes collected in March 1996 on the last day that frogs were seen in large numbers along the stream. In January 1997 dead frogs started to appear at Taponga River. By march 1997, ten frogs had been found dead, this was the first time any dead frogs had been found since monitoring commenced at this site in 1993. Several dead frogs were also found at two other Victorian sites. In March 1997, twelve sub-adults and five recently metamorphosed frogs were collected from each of the Taponga River and Goulburn River populations. The subadults were placed in captivity and the metamorphs were preserved for histological examination. Subsequent captive deaths as well as histology results indicated an infection level of about 20% in each population.

Monitoring since 1997 has failed to detect any population decline at either site. These observations indicate that the impact of chytrids at a population level is influenced by other demographic and/or environmental factors. This paper presents an overview of the population dynamics of L. spenceri over the past eight years as well as the observed impact of, and possible influences on, the chytrid fungus.